The present invention relates to a magneto-optical recording medium for conducting recording or reading-out by irradiation with laser beams from the film surface side of the recording medium by using a flying head equipped with an objective lens having a large NA value.
In recent years, optical recording apparatus for recording a large amount of data at a high density and rapid recording or reading-out thereof have been noted with the development of multimedia. As the optical recording apparatus, there are those using a disc intended only for reading-out which permits only reading-out of information because it is produced by stamping of the information on the disc, such as CD and Laser Discs; those using a direct read after write disc which permits recording only once, such as CD-R; and those using a rewritable disc which permits rewriting and deletion of data any number of times by the use of a magneto-optical recording method or a phase-change recording method. Of these optical recording apparatus, magneto-optical recording apparatus are mainly used in fields where a high transfer rate is required. Data are read out or recorded by using a light spot obtained by focusing laser beams to the limit of diffraction by the use of a lens. When the wavelength of the laser beams is taken as xcex and the numerical aperture of the lens as NA, the size of the light spot is about xcex/NA (Yoshihito Tsunoda (a chief editor) xe2x80x9cFundamentals and Practice of Optical Disc Storagexe2x80x9d, The Institute of Electronic Information and Communication of Japan (1995), p.65). For recording or reading out a higher-density pattern, i.e., a smaller pattern, a smaller laser beam spot is required. For reducing the size of the light spot, two methods, i.e., a method of reducing the wavelength of the laser beams (xcex) and a method of increasing the numerical aperture (NA) of the lens are thought of on the basis of the above expression. When the half angle of diaphragm is taken as xcex8, the numerical aperture (NA) of the lens is expressed by the equation: NA=sin xcex8 and has a value of less than 1. NA of an objective lens used in an optical recording apparatus put to practical use is at most about 0.6. This is because the coma and astigmatism caused when the optical axis of the objective lens is tilted relative to the substrate become remarkable with an increase of NA. For solving this problem, it is important to reduce the thickness of a light-transmittable transparent layer (corresponding to the thickness of a substrate in a conventional optical recording apparatus) and prevent the optical axis of the objective lens from tilting relative to the substrate. Therefore, the following optical recording medium and a process for production thereof have been disclosed: recording or reading-out is conducted by pressing a slider equipped with an objective lens and the like against the surface of the transparent resin layer of a recording medium comprising a substrate, grooves or pits formed on the substrate for obtaining signals at the time of tracking, address signals and the like, a recording layer including a reflective layer and formed on the substrate having the grooves or pits, and the transparent resin layer of about 0.1 mm in thickness formed on the recording layer (Japanese Patent Kokai No. 8-235638). This optical recording medium is disadvantageous in that the transparent resin layer is liable to be injured by the sliding of the slider on the transparent resin layer because the slider is moved while being pressed against the transparent resin layer. The transparent resin layer on the reflective film and the recording film is formed by spin-coating an ultraviolet-setting resin or adhering a transparent resin film. When the transparent resin film is adhered, the adhesion has to be carried out so that no air accumulation portion or bubble may be formed by the intrusion of air into the space between the resin film and the recording film. Therefore, the productivity is not sufficient. Even when a layer of the ultraviolet-setting resin of about 0.1 mm in thickness is formed, this thickness is approximately 5 to 10 times the thickness of 10 to 20 xcexcm of the protective film of a conventional recording media. Therefore, the viscosity of the resin has to be set at a considerably high value. In this case, there are problems such as the insufficient uniformity of the transparent resin layer and the formation of air bubbles by the intrusion of air, so that the productivity and the yield are not sufficient as in the case of the adhesion of the transparent film.
On the other hand, a near-field optical recording and reading-out method has been proposed in which NA of an objective lens is effectively increased by using near-field light by casting light on a recording medium through a solid hemispherical lens (SIL) to form an image on the recording medium and adjusting the distance between SIL and the surface of the optical recording medium to about xc2xcxcex (xcex: laser wavelength) (U.S. Pat. No. 5,125,750). Even in this technique, when the laser wavelength is 650 nm, the distance between the underside of SIL and the surface of the recording medium is as narrow as about 160 nm, so that there are problems such as scratches of the recording medium surface given by SIL.
The present invention was made in order to remove the defects of the above-mentioned prior arts and is intended to provide a film-surface light-introduction type magneto-optical recording medium which is used by means of an objective lens having a large NA value and is excellent in resistance to sliding, productivity and yield, and a magneto-optical recording apparatus.
In order to achieve the above object, there is provided as a first aspect of the present invention a magneto-optical recording medium for conducting recording or reading-out from the film surface side of the recording medium by traveling of a flying head incorporated with an objective lens having a numerical aperture (NA) of not more than 0.95 and not less than 0.8, wherein at least a reflective film, a recording layer, a transparent dielectric layer, a transparent resin layer and a lubricant layer are laminated on a substrate in that order.
As a second aspect of the present invention, there is provided a magneto-optical recording apparatus which uses a magneto-optical recording medium obtained by laminating at least a reflective film, a recording layer, a transparent dielectric layer and a lubricant layer on a substrate in that order, and in which a flying head incorporated with an objective lens having a numerical aperture (NA) of not more than 0.95 and not less than 0.8 is located so as to face the film surface side of the magneto-optical recording medium. Employment of the above structure makes it possible to attain a much higher packing density than before.